In pipelines for transporting oil and natural gas over long distances, improvements are being made in the efficiency of transport by increasing pressures and the pipelines are being laid in deeper seas. For this reason, the electric resistance welded pipe which is used for the pipelines is being required to be made thicker and higher in strength. Further, when laying pipelines in deeper seas, the electric resistance welded pipe is sometimes subjected to repeated bending and unbending, so a lower yield ratio is demanded to prevent buckling.
If electric resistance welded pipe becomes thicker, the processing strain which is introduced when producing electric resistance welded pipe from hot rolled steel plate becomes larger. For this reason, a rise in yield ratio becomes harder to suppress. The yield ratio is an indicator of the durability when stress larger than the yield stress is applied to the material and therefore the material yields, then buckles or breaks. The lower the yield ratio, the harder it is for the steel pipe to buckle. The yield ratio (below, also referred to as “YR”) is a value which is expressed by the ratio (YS/TS) of the yield stress (below, also referred to as “YS”) and tensile strength (below, also referred to as “TS”).
In general, it is known that if making the metal microstructure of the steel material a dual phase microstructure comprised of soft phases and hard phases, the YR falls. Electric resistance welded pipe with a base material of a metal microstructure which is made a dual phase microstructure has been proposed.
PLT 1 discloses low yield ratio electric resistance welded pipe where second phases constituted by island shaped martensite and retained austenite are formed. PLT 2 discloses low yield ratio hot rolled steel plate to be used as a material for line pipe which is produced by spiral pipemaking or UO pipemaking.